In any internal combustion engine, power is the product of how much "push" you have (Torque) and how fast you are applying it (RPM). While it seems logical that spinning an engine faster should always result in more power, there is a physical "ceiling" where the torque begins to drop so rapidly that the increasing speed can no longer compensate for the loss.

Let's take Perkins 400 Series 402D-05 Industrial Engine chart as an example,

Phase 1: The torque climb (1,500 – 2,400 rpm)

In this early stage, the engine is finding its strength, torque rises from roughly 25 Nm at 1,500 rpm to its peak of 29.7 Nm at 2,400 rpm.                                                                                                                                                                                                                                                     

=> As the engine speeds up, the volumetric efficiency improves. More air and fuel are entering the cylinders effectively, creating a stronger combustion "push" on the pistons. Power is rising steadily during this phase because both the torque (strength) and RPM (speed) are increasing simultaneously.

Phase 2: The peak - plateau (2,400 – 3,000 rpm)

This is the "sweet spot" for high-efficiency operation.

Torque hits its maximum of 29.7 Nm at 2,400 rpm and stays almost perfectly flat until 3,000 rpm.

=> Even though the engine is spinning faster, the torque isn't dropping yet, because Power (P) = Torque (τ) × Angular Velocity (ω), the flat torque curve allows the power curve to keep climbing at a steep, linear rate.This is the most stable range for heavy industrial work.

Phase 3: The Power Drop-off Transition (3,000 – 3,600+ rpm)

Between 3,000 and 3,600 rpm, the torque begins to decline,dropping toward 27 Nm,  and can no longer compensate for speed because:

• At very high RPMs, the intake valves are opening and closing so rapidly that the air doesn't have enough time to rush into the cylinder completely.
• Diesel fuel takes a specific amount of time to ignite and burn, for that at extremely high RPMs, the piston may actually be moving downward faster than the expanding gases can push it. If the combustion isn't finished by the time the exhaust valve opens, that energy is simply wasted out of the tailpipe instead of being turned into power.
• As the engine spins faster, the friction between the pistons, bearings, and crankshaft increases exponentially, the engine begins using a massive chunk of its own generated power just to overcome its internal resistance.
  
  

The performance of the Perkins 402D-05 illustrates that an engine's output isn't just about how fast it spins, but how effectively it can maintain its "pushing" strength (torque) at those speeds. Once the engine passes 3,000 rpm, the decline in torque begins to slow the growth of the power curve. When the engine hits 10.2 kW at 3,600 rpm, it has reached its mechanical limit; beyond this point, the torque can no longer compensate for the high speed, leading to an inevitable drop in total power.